Develop Reference

How to run code (Google/Doubleclick Ads) without block main thread

Is there any way to run Google Ads code without block main thread? Google Pagespeed Insights shows me a warning "Reduce the impact of third-party code": Third-party code blocked the main thread for ...
Third-Party Size Main-Thread Blocking Time
Google/Doubleclick Ads 193 KB 253 ms
I've placed a script to the end of the page in the footer.
<script data-ad-client="ca-pub-xxx" async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>
I tried to add "data-aload data-original=..." but it doesn't help. Maybe it would a right choice to use requestAnimationFrame() or setTimeOut(), but I don't know how to implement it on this.

You can add script dynamically. NB there is no need to add async since browser considers all dynamic script async by default
const loadScript = (src, id, callback) => {
const script = document.createElement('script');
script.src = src; // URL for the third-party library being loaded.
script.id = id; // e.g., googleMaps or stripe
script.defer = true; // make sure that browser will run script after page loaded
document.body.appendChild(script);
script.onload = () => {
if (callback) callback(); // conditional callback
};
};

Using script tag to pass arguments to JavaScript

I need to implement a cross-site comet http server push mechanism using script tag long polling. (phew...) For this, I dynamically insert script tags into the DOM and the server sends back short js scripts that simply call a local callback function that processes the incoming messages. I am trying to figure out a way to associate each one of these callback calls with the script tag that sent it, to match incoming replies with their corresponding requests.
Clearly, I could simply include a request ID in the GET url, which is then returned back in the js script that the server generates, but this creates a bunch of unnecessary traffic and doesn't strike me as particularly elegant or clever.
What I would like to do is to somehow associate the request ID with the script tag that I generate and then read out this request ID from within the callback function that is called from inside this script tag. That way, all the request management would remain on the client.
This leads me to the following question: Is there a way to ask the browser for the DOM element of the currently executing script tag, so I can use the tag element to pass arguments to the contained javascript?
I found this thread:
Getting the currently executing, dynamically appended, script tag
Which is asking exactly this question, but the accepted answer isn't useful to me since it still requires bloat in the server-returned js script (setting marker-variables inside the script) and it relies on unique filenames for the scripts, which I don't have.
Also, this thread is related:
How may I reference the script tag that loaded the currently-executing script?
And, among other things, suggests to simply grab the last script in the DOM, as they are executed in order. But this seems to only work while the page is loading and not in a scenario where scripts are added dynamically and may complete loading in an order that is independent of their insertion.
Any thoughts?
PS: I am looking for a client-only solution, i.e. no request IDs or unique callback function names or other non-payload data that needs to get sent to and handled by the server. I would like for the server to (theoretically) be able to return two 100% identical scripts and the client still being able to associate them correctly.

I know you would like to avoid discussions about changing the approach, but that's really what you need to do.
First, each of the script tags being added to the DOM to fire off the poll request is disposable, i.e. each needs to be removed from the DOM as soon as its purpose has been served. Else you end up flooding your client DOM with hundreds or more dead script tags.
A good comparable example of how this works is jsonp implementations. You create a client-side named function, create your script tag to make the remote request, and pass the function name in the request. The response script wraps the json object in a function call with the name, which then executes the function on return and passes the json payload into your function. After execution, the client-side function is then deleted. jQuery does this by creating randomly generated names (they exist in the global context, which is really the only way this process works), and then deletes the callback function when its done.
In regards to long polling, its a very similar process. Inherently, there is no need for the response function call to know, nor care, about what script tag initiated it.
Lets look at an example script:
window.callback = function(obj){
console.log(obj);
}
setInterval(function(){
var remote = document.createElement('script');
remote.src = 'http://jsonip.com/callback';
remote.addEventListener('load', function(){
remote.parentNode.removeChild(remote);
},false);
document.querySelector('head').appendChild(remote);
}, 2000);​
This script keeps no references to the script elements because again, they are disposable. As soon as their jobs are done, they are summarily shot.
The example can be slightly modified to not use a setInterval, in which case you would replace setInterval with a named function and add logic into the remote load event to trigger the function when the load event completes. That way, the timing between script tag events depends on the response time of your server and is much closer to the actual long polling process.
You can extend this even further by using a queueing system to manage your callbacks. This could be useful if you have different functions to respond to different kinds of data coming back.
Alternatively, and probably better, is to have login in your callback function that handles the data returned from each poll and executes whatever other specific client-side logic at that point. This also means you only need 1 callback function and can get away from creating randomly generated callback names.
If you need more assistance with this, leave a comment with any specific questions and I can go into more detail.

It's most definitely possible but you need a little trick. It's a common technique known as JSONP.
In JavaScript:
var get_a_unique_name = (function () {
var counter = 0;
return function () {
counter += 1;
return "function_" + counter;
}
}()); // no magic, just a closure
var script = document.createElement("script");
var callback_name = get_a_unique_name();
script.src = "/request.php?id=12345&callback_name=" + callback_name;
// register the callback function globally
window[callback_name] = function (the_data) {
console.log(the_data);
handle_data(the_data); // implement this function
};
// add the script
document.head.appendChild(script);
The serverside you can have:
$callback_name = $_GET["callback_name"];
$the_data = handle_request($_GET["id"]); // implement handle_request
echo $callback_name . "(" . json_encode($the_data) . ");";
exit; // done
The script that is returened by /request.php?id=12345&callback_name=XXX will look something like this:
function_0({ "hello": "world", "foo" : "bar" });

There may be a solution using onload/onreadystate events on the script. I can pass these events a closure function that carries my request ID. Then, the callback function doesn't handle the server reply immediately but instead stores it in a global variable. The onload/onreadystate handler then picks up the last stored reply and tags it with the request ID it knows and then processes the reply.
For this to work, I need to be able to rely on the order of events. If onload is always executed right after the corresponding script tag finishes execution, this will work beautifully. But, if I have two tags loading simultaneously and they return at the same time and there is a chance that the browser will execute both and afterwards execute botth onload/onreadystate events, then I will loose one reply this way.
Does anyone have any insight on this?
.
Here's some code to demonstrate this:
function loadScript(url, requestID) {
var script = document.createElement('script');
script.setAttribute("src", url);
script.setAttribute("type", "text/javascript");
script.setAttribute("language", "javascript");
script.onerror = script.onload = function() {
script.onerror = script.onload = script.onreadystatechange = function () {}
document.body.removeChild(script);
completeRequest(requestID);
}
script.onreadystatechange = function () {
if (script.readyState == 'loaded' || script.readyState == 'complete') {
script.onerror = script.onload = script.onreadystatechange = function () {}
document.body.removeChild(script);
completeRequest(requestID);
}
}
document.body.appendChild(script);
}
var lastReply;
function myCallback(reply) {
lastReply = reply;
}
function completeRequest(requestID) {
processReply(requestID, lastReply);
}
function processReply(requestID, reply) {
// Do something
}
Now, the server simply returns scripts of the form
myCallback(message);
and doesn't need to worry at all about request IDs and such and can always use the same callback function.
The question is: If I have two scripts returning "simultaneously" is it possible that this leads to the following calling order:
myCallback(message1);
myCallback(message2);
completeRequest(requestID1);
completeRequest(requestID2);
If so, I would loose the actual reply to request 1 and wrongly associate the reply to request 2 with request 1.

It should be quite simple. There is only one script element for each server "connection", and it can easily be stored in a scoped, static variable.
function connect(nameOfCallback, eventCallback) {
var script;
window[nameOfCallback] = function() { // this is what the response invokes
reload();
eventCallback.call(null, arguments);
};
reload();
function reload() {
if (script && script.parentNode)
script.parentNode.removeChild(script);
script = document.createElement(script);
script.src = "…";
script.type = "text/javascript";
document.head.appendChild(script);
// you might use additional error handling, e.g. something like
// script.onerror = reload;
// but I guess you get the concept
}
}

Stop Javascript and HTML from Loading From Cache

I am building a single page javascript app and when the application starts I use a single javascript file to load every other file I need on the fly. When I hit refresh, according to firebug, my HTML page as well as javascript pages will load with a 304 Not Modified Error and my javascript stops working.
I understand this is due to browser caching, but how can I avoid this? I load the initial HTML page with a single script call
<script src="js/config.js" type="text/javascript"></script>
and then continue to load the rest dynamically from within that script
window.onload = function () {
var scripts = ['http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.7.1.min.js', 'js/core.js', 'js/sandbox.js']; //Application scripts
var loaded = 0;
//Callback is executed after all scripts have been loaded.
var callback = function () {
if (loaded + 1 == scripts.length) {
//Create Modules
CORE.loader("js/modules/Login.js", function () {
CORE.createModule('loginForm', Login);
});
//Create HTML bindings.
CORE.createBinding('appContainer', '#Login', 'login.html');
CORE.bindHTML(window.location.hash); //Loads hash based page on startup
} else {
loaded++;
loadScript(scripts[loaded], callback);
}
};
loadScript(scripts[0], callback);
function loadScript(scriptSrc, callback) {
var script = document.createElement('script');
script.type = 'text/javascript';
script.async = true;
script.src = scripts[loaded];
if (script.readyState) {
script.onreadystatechange = function () {
if (script.readyState == 'loaded' || script.readyState == 'complete') {
script.onreadystatechange = null;
callback();
}
};
} else {
script.onload = function () {
callback();
};
}
document.getElementsByTagName('head')[0].appendChild(script);
}
};
I know that Gmail uses cookies to prevent this. Does anyone have any idea how to take that approach? Should I set the cookie on the server and then check it with JS on each page load/refresh and use something like window.location.refresh() if the cookie tells me the page is loaded from cache?

To expand on #Ramesh's answer:
to force a reload of the js file, instead of the cache, use this html:
<script src="js/config.js?v=42" type="text/javascript"></script>
The next time you make changes to that file just +1 the v. This also works with css files by the way.

Caching is an important for performance reasons. I would recommend you pass a version number in your query string and with every update, increment the version number. This will force the browser to resend the request and it will load from cache if it already has the same version.

I agree with all the other answers. 304 is not an error and there are many reasons why this behavior is correct.
That being said, there is a simple "hack" you can use. Simply attach a unique URL parameter to the JS call.
var timestamp = +new Date;
var url = "http://mysite.com/myfile.js?t=" + timestamp;
Again, this is a hack. Performance wise, this is horrible.

<META HTTP-EQUIV="PRAGMA" CONTENT="NO-CACHE">
Add this into your HTML HEAD.

<?php
$xn = date("YmdHis");
echo " <script src='root.js?$xn'></script>";
?>

I would suggest to use Javascript to generate a random number using Math.random multiply it and then Math.floor to return the integer of it. Then, I would add this number to the URL as a variable. Since the number changes during every page load, the file should never be cached.
<script>
var url="yourscript.js";
var extra="?t=";
var randomNum = String((Math.floor(Math.random() * 200000000000000)));
document.getElementById('myScript').src = url+extra+randomNum;
</script>
<script id="myScript"></script>

You need to set script.src = scripts[loaded]; after adding the onreadystatechange/onload handlers. Otherwise the event is going to fire before the handlers are added, since the cached version loads instantly.

After struggling with the cache issue for months, trying just about anything (including a script which changes the URLs using a parameter) I found a post which explains how to do that using the IIS.
Start IIS Manager (INETMGR from the start menu works for me)
Navigate to desired site in the Connections tree
Open Output Caching
Edit Feature Settings
Uncheck Enable cache and Enable kernel cache
If you cannot save changes, make sure your web.config file is not marked read only
IISRESET is required for the changes to take place
This is the original post, which mentions that it is relevant for IIS 7.5 (in Windows 7)
https://forums.iis.net/t/959070.aspx?How+do+you+disable+caching+in+IIS+
One of the things I tried before that was adding .html and .js in Output Caching, and checking "Prevent All Caching". So if it doesn't work after the IISRESET, try that, though I'm not sure it is actually required.
EDIT:
If it does not work, still in the IIS go to HTTP Response Headers and add new actions:
Name: cache-control, Value: no-cache
Name: expires, Value: 0

Load JavaScript dynamically [duplicate]

This question already has answers here:
JQuery to load Javascript file dynamically
(2 answers)
Closed 7 years ago.
I have a web page and a canvas with Google Maps embedded in it. I am using jQuery on this site.
I want to load Google Maps API only if the user clicks on "Show me the map". Further, I want to take away the whole loading of the Google Maps from the header in order to improve my page performance.
So I need to load JavaScript dynamically. What JavaScript function I can use?

You may want to use jQuery.getScript which will help you load the Google Maps API javascript file when needed.
Example:
$.getScript('http://maps.googleapis.com/maps/api/js?libraries=geometry&sensor=true', function(data, textStatus){
console.log(textStatus, data);
// do whatever you want
});

Use the Loading on Demand Loading Strategy
Loading on Demand
The previous pattern loaded additional JavaScript unconditionally after page load, assuming
that the code will likely be needed. But can we do better and load only parts of
the code and only the parts that are really needed?
Imagine you have a sidebar on the page with different tabs. Clicking on a tab makes an
XHR request to get content, updates the tab content, and animates the update fading
the color.
And what if this is the only place on the page you need your XHR and animation
libraries, and what if the user never clicks on a tab?
Enter the load-on-demand pattern. You can create a require() function or method that
takes a filename of a script to be loaded and a callback function to be executed when
the additional script is loaded.
The require() function can be used like so:
require("extra.js", function () {
functionDefinedInExtraJS();
});
Let’s see how you can implement such a function. Requesting the additional script is
straightforward. You just follow the dynamic element pattern. Figuring out
when the script is loaded is a little trickier due to the browser differences:
function require(file, callback) {
var script = document.getElementsByTagName('script')[0],
newjs = document.createElement('script');
// IE
newjs.onreadystatechange = function () {
if (newjs.readyState === 'loaded' || newjs.readyState === 'complete') {
newjs.onreadystatechange = null;
callback();
}
};
// others
newjs.onload = function () {
callback();
};
newjs.src = file;
script.parentNode.insertBefore(newjs, script);
}
“JavaScript Patterns, by Stoyan Stefanov
(O’Reilly). Copyright 2010 Yahoo!, Inc., 9780596806750.”

you would just generate the script tag via javascript and add it to the doc.
function AddScriptTag(src) {
var node = document.getElementsByTagName("head")[0] || document.body;
if(node){
var script = document.createElement("script");
script.type="text/javascript";
script.src=src
node.appendChild(script);
} else {
document.write("<script src='"+src+"' type='text/javascript'></script>");
}
}

I think you're loking for this http://unixpapa.com/js/dyna.html
<input type="button" onclick="helper()" value="Helper">
<script language="JavaScript">
function helper()
{
var head= document.getElementsByTagName('head')[0];
var script= document.createElement('script');
script.type= 'text/javascript';
script.src= 'your_script_url';
head.appendChild(script);
}
</script>

I used the Tangim response, but after found that is more easy use jquery html() function. When we make ajax request to html file that have html+javascript we do the follow:
$.ajax({
url:'file.html',
success:function(data){
$("#id_div").html(data); //Here automatically load script if html contain
}
});

Loading scripts dynamically

I'm loading a few YUI scripts dynamically in my code in response to an Ajax request. The DOM and the page is fully loaded when the request is made - it's a response for an user event.
I add the <scripts> tag to head as children, but I stumbled in a few problems:
I add two YUI scripts hosted at the Yahoo! CDN and an inlined script of my own responsible for creating object, adding event listeners and rendering the YUI widgets. But I when my script run the YUI scripts are not loaded yet giving me errors and not running as I expect.
There's a way to only run my script (or define a function to be run) when YUI scripts are fully loaded?

Have you tried an onload event?
Edited:(thanks Jamie)
var script = document.createElement("script");
script.type = "text/javascript";
script.src = src;
//IE:
if(window.attachEvent && document.all) {
script.onreadystatechange = function () {
if(this.readyState === "complete") {
callback_function(); //execute
}
};
}
//other browsers:
else {
script.onload = callback_function; //execute
}
document.getElementsByTagName("head")[0].appendChild(script);

If you're using YUI 2.x I highly recommend using the YUI Get utility, as it's designed to handle just this sort of a problem.

If you are loading multiple individual script files from the Yahoo! CDN, you'll need to makes sure both are loaded before executing your dependent code. You can avoid this using the combo handler. See the Configurator to get what the script url should be to load both/all needed YUI files from one url.
http://developer.yahoo.com/yui/articles/hosting/
With that in mind, assuming you must load the YUI files asynchronously, you should use an onload/onreadystatechange handler as noted by digitalFresh.
I would recommend the following pattern, however:
(function (d) {
var s = d.createElement('script'),
onEvent = ('onreadystatechange' in s) ? 'onreadystatechange' : 'onload';
s[onEvent] = function () {
if (("loaded,complete").indexOf(this.readyState || "loaded") > -1) {
s[onEvent] = null;
// Call your code here
YAHOO.util.Dom.get('x').innerHTML = "Loaded";
}
};
// Set the src to the combo script url, e.g.
s.src = "http://yui.yahooapis.com/combo?2.8.1/...";
d.getElementsByTagName('head')[0].appendChild(s);
})(document);

You could use a setTimeout() to run some function that just checks if it's loaded - check something like
if (typeof YUI_NAMESPACED_THING !== "undefined") runCode()
EDIT Thanks, CMS

If I understand this correctly, your ajax response with this:
<script href="yui-combo?1"></script>
<script href="yui-combo?2"></script>
<p>some text here</a>
<script>
// using some of the components included in the previous combos
// YAHOO.whatever here...
</script>
If this is the case, this is a clear case in which you should use dispatcher plugin. Dispatcher will emulate the browser loading process for AJAX responses. Basically it will load and execute every script in the exact order.
Best Regards,
Caridy